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Your Phone Is Locked. Just Drive.

The statistics on distracted driving are pretty scary. Just making cellphone calls increases your chances of crashing by four times; sending text messages increases the risk 23 times.

We know this, we get this, but we keep doing it. About half of all teenagers admit to texting while driving, for example, no matter how many statistics and horror stories we pass along to them.

If you’re a concerned parent or employer, therefore, you may want to consider fighting technology with technology. There’s a new category of cellphone apps made just for this purpose: text blockers like iZup, tXtBlocker, CellSafety and ZoomSafer. When your car is in motion, they lock up your phone so you can’t text, call, e-mail or surf the Web.

How do they know when you’re driving? They rely on your phone’s GPS to calculate your speed. If it’s more than five or 10 miles an hour, it’s pretty clear that you’re no longer walking. (You could be riding your bicycle, of course. But come to think of it, that’s probably not a great time to be texting, either.)

You’ll know when the software is in effect: your screen is covered by a “MESSAGES BLOCKED” screen. Incoming calls go directly to voice mail; incoming text messages don’t appear until you stop driving.

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A number of apps use a phone’s GPS to determine when someone is driving, then lock certain features. The tXtBlocker app lets you designate “no phone” areas, like schools.

They all let you dial 911, and they all let you set up certain phone numbers in advance (like your parents’) that work even when everything else is blocked. But otherwise, you quickly realize that you’re wasting your time trying to bypass the blockade, and you focus on getting where you’re going so you can get back to your phone. Which, of course, is the whole idea.

The four apps are very similar — they all drain your battery faster, they all take a couple of minutes to “notice” that you’re moving. But there are some differences, in features, philosophy and the Passenger Problem. (Which is, How can I bypass the block if I’m not the one driving?)

TXTBLOCKER If you, O parent, were born with a dominant Big Brother gene, this is the app for you. It prevents texting, calling, e-mailing or Web surfing when the phone is in motion. It also lets you pinpoint the phone’s location on a map online, so you can track your child’s comings and goings.

You can set it up so that you get a text message every time the phone’s owner exceeds, say, 65 miles an hour. (An unlimited texting plan might be useful.)

You can also define a “no phone” zone, like your young hellion’s school; the phone stops working inside this area.

As for the Passenger Problem, you can temporarily unblock the phone by solving a timed puzzle.

In the blessing/curse department, note that tXtBlocker “pings” your phone’s GPS only once every several minutes. As a result, it’s slow to block and unblock the phone — but it doesn’t burn through your phone’s battery as fast as its rivals. (Price: $25, plus $10 a month or $100 a year; family plan available; for BlackBerry and Windows Mobile. Android and iPhone versions coming soon.)

The Passenger Problem: if you click Request Permission on the blockade screen, your parent or boss gets a text message. If your overlord approves of your request by replying with “unblock,” you can use the phone. (Price: $10 a month, on Android, BlackBerry and Nokia phones.)

IZUP If CellSafety is a friendly babysitter, iZup is the distraction Nazi. Once you’re in motion, iZup turns your phone into a brick. You don’t have access to a single feature (except one single designated app, intended for your GPS navigation program).

“IZUP does not allow a user to select some distractions and stop others,” says the company. “We believe that is key in the fight against distracted driving. We are purists in our approach here.”

You can’t even whip out your phone at a red light or stop sign; to guard against that sort of sneakiness, iZup doesn’t reactivate for several minutes after you stop.

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ZoomSafer, which can be harsh or lenient, has a flat $25 fee option.

You can’t make any calls except to 911 or one of three preapproved phone numbers. Only the parent or boss can unlock the blocking — by entering a password before you start to drive. If you’re a passenger, tough rocks.

One more note: maybe these apps are young, or maybe the technology gods had a rough week — but in my testing, three of the four apps stopped working at one point or another. TXtBlocker, CellSafety and ZoomSafer each worked intermittently until company reps walked me through reinstalling them. Only iZup worked every time, all the time, solid as a rock.(Price: $5 a month or $50 a year; family plan available; BlackBerry, some Android, LG and Samsung phones.)

ZOOMSAFER When ZoomSafer kicks in, the phone makes a little sound. You can replace that sound with one you’ve recorded yourself; your toddler saying, “Stay alive, mommy!” or “Come home soon, daddy!” makes a particularly effective choice.

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On your Web-based account page, you can specify all kinds of options. You can permit or block incoming calls, permit or block numbers that you dial by voice, permit a passenger to open a timed interval of phone freedom or set up an automatic reply to unanswered texts and e-mail messages (“I’m driving right now; I’ll get back to you”).

In other words, you can set up ZoomSafer to be either an unfeeling obelisk like iZup or a lenient pal like CellSafety.

The company also offers another app that lets you listen and respond to e-mail by voice as you drive. (Wait a sec — isn’t the whole idea to eliminate distractions? Whose side is this company on, anyway?)

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CellSafety lets you make phone calls, but you can't text or browse.

Maybe the best thing about ZoomSafer, though, is that instead of paying its monthly $3 fee, you can pay a one-time $25. The other apps here require a monthly fee forever, which is borderline outrageous. (ZoomSafer is for BlackBerry and some Windows Mobile phones.)

Even ZoomSafer, however, is not the perfect solution to the distracted-driving problem.

For starters, these apps also lock up your phone when you’re on a train or bus, which is a tad unnecessary.

Furthermore, they work only on GPS-equipped smartphones, mainly BlackBerrys and Android phones. (Why not the iPhone? Because the current iPhone software doesn’t permit a program to run in the background. This summer’s 4.0 software will presumably solve that problem.)

In other words, text blockers won’t run on the hundreds of millions of regular dumbphones.

There are apps, like one called Cell Control, that don’t rely on the phone’s GPS. Instead, they communicate with a small transmitter that you plug into your car’s OBDII jack. (This jack is usually under the driver-side dashboard.)

These programs work with many nonsmartphones and even laptops, and can control several phones in the same car. And they don’t eat up your battery as fast as the software-only versions.

Unfortunately, Cell Control is also much more expensive, because you have to buy a transmitter ($90) and the app ($25) and pay a monthly fee ($8). And, of course, this kind of system has no effect on your phone when you’re in someone else’s car.

In any case, it might be worth considering a text-blocking app for your teenager — or even for yourself, if only to make your phone so inconvenient while driving that you won’t bother with it. Because the world already has enough horrifying driving-distraction statistics; you should avoid becoming one yourself.

E-mail: pogue@nytimes.com.

A version of this article appears in print on April 29, 2010, on Page B1 of the New York edition with the headline: Your Phone Is Locked. Just Drive. Order Reprints|Today's Paper|Subscribe